The overall goal of this research is to understand how children hear in complex auditory environments. Spatial hearing, the ability to understanding speech in background noise and suppression of interference from echoes (precedence effect; PE) will be studied in children with normal hearing and in deaf children who received bilateral cochlear implants (BI-CIs). Aim 1 is to investigate the maturational progression of spatial hearing in complex acoustic environments, and the degradation that noise and echoes have on these abilities. We will test the hypothesis that spatial hearing skills are well developed early in life under simple conditions but not under complex, challenging conditions, such as when competing signals, echoes or noise are present. Aim 2 is to investigate Speech intelligibility in a multi-source environment in normal-hearing children. The decrease in masking due to spatial separation of the target and interferers (spatial release from masking; SRM) will be the focus of this work. We will test hypotheses regarding the maturation of centrally- mediated auditory abilities during childhood. Studies in Aims 1 and 2 will also provide important benchmarks for understanding the significance of bilateral benefits in children with hearing loss, and in particular for a growing population of children with bilateral cochlear implants (BI-CIs). In Aim 3, using measures from Aims 1 and 2, benefits of BI-CIs will be studied in groups of children that vary according to auditory experience prior to, and following, activation of their second implant. It is hypothesized that children with simultaneous BI-CIs will be on a quicker trajectory for emergence of bilateral benefits than children with sequential BI-CIs; that differences will be significantly diminished after 2 years of bilateral experience; that children with post- lingual deafness will perform better than those with pre-lingual deafness due to early establishment of binaural circuitry. The proposed work will help us to better understand the development of functions that are important in everyday situations. The measures developed here can be eventually implemented in clinical evaluations of amplification and fitting strategies. Findings from the BI-CI studies will provide quantitative measures of bilateral benefit in children and will be enlightening about the role of auditory plasticity and experience in a young human population.